1. Field of the Invention
The present invention relates to a digital wireless receiving apparatus, and in particular to a wireless apparatus which is used for digital mobile communication.
In recent years, with a stringent condition of frequency resources in wireless communication, a high-efficiency transmission system by digitalization has been generally used. On the other hand, for a channel occupation bandwidth, a narrow one has been demanded.
This demand has been achieved, on a transmitter side, by reducing a roll-off factor and by adopting a linearization art while on a receiver side, an analog filter which is superior in an attenuation characteristic has been developed in order to reduce an interfering wave between adjacent channels. In addition, for the improvement of performance a removal of the interfering wave has been recently attempted with a digital filter.
2. Description of the Related Art
FIG. 22 shows a prior art digital wireless receiving apparatus which uses an analog filter and a digital filter as mentioned above. In this prior art, after being amplified at an RF (Radio Frequency) amplifier 32, an RF reception signal from an antenna (ANT) 31 is transformed into an intermediate frequency (IF) signal (a base band signal) at a mixer 33 with a local oscillation signal from a local oscillator 34. Then the intermediate frequency signal is amplified at an IF amplifier 35 and sent to an IF filter 101. The above components 31-35 form a high frequency portion 100.
The IF filter 101 secures a pass band characteristic in the base band signal, removes an interfering wave and sends the base band signal to a quadrature demodulator 102. In the quadrature demodulator 102, by using a regenerated carrier wave 90, I/Q quadrature components are separated from the base band signal at a quadrature detector 9 and are transformed into digital signals at A/D converters 11-1, 11-2. Then, the digital signals secure the pass band characteristic for demodulation at digital filters (FIR) 12-1, 12-2, and are taken over by an identification portion 103 for the identification operation.
In such a prior art apparatus, the IF filter 101 uses an analog filter with a high selectivity (a high attenuation characteristic) to satisfy a selection characteristic between adjacent channels where the interfering wave exists. Accordingly, the analog filter becomes large in shape and expensive. Also, it is technically difficult to realize an analog filter with a narrow band and a high attenuation characteristic, and is not practical to mount such an analog filter on a base station, especially on a mobile station (a portable device or a car-mounted device).
To avoid such a defect, the IF filter 101 has to use a cheep and small-sized analog filter with a normal pass band and attenuation characteristic. On the contrary, if a demodulator (a base band portion) uses digital filters 12-1, 12-2 which are composed of a DSP or the like to satisfy the above mentioned characteristic (a narrow band and a high attenuation characteristic), a tap number of the digital filter has to be increased, so that and a delay quantity for the demodulation can not be neglected, which is not practical either.
In addition, in a system which provides an adjacent channel selection characteristic with the digital filter there is a tendency that a dynamic range for securing the characteristic becomes small, compared with a system which achieves the same with the prior art analog filter.
To avoid such a defect, after the high frequency portion removes the interfering wave to some extent with a small-sized analog filter, the demodulator has to use the digital filter with a small delay quantity so that the whole receiving system may satisfy the adjacent channel selection characteristic.
In this case, as an attenuation amount of an interfering wave between adjacent channels is greatly influenced by a linear characteristic of a pre-stage circuit of the digital filter, by providing an AGC circuit in the RF amplifier or the like of the pre-stage circuit of the digital filter the linear characteristic has to be secured and the adjacent channel selection characteristic has to be made advantageous.
According to an arrangement shown in FIG. 23, a case will now be considered where an AGC control is executed from the IF filter 101 to the RF amplifier 32 through a level detector 41 and an AGC controller 42.
If a desired electric field wave is inputted from the antenna 31, an output electric field of the IF filter 101 shows two cases where the AGC control is executed and where no AGC control is executed, as shown in FIGS. 24A, 24B respectively.
Assuming that there is an interfering wave which is larger by 60 dB than the desired wave in the adjacent channel and that the removal performance for the adjacent channel of the IF filter 101 is 20 dB, the detected electric fields in FIGS. 24A, 24B become those as shown in FIGS. 25A, 25B respectively by the influence of the interfering wave. That is, if the AGC control system is ruled by the interfering wave and the interfering wave increases in magnitude, the execution of the AGC control will lead desensitization restraining an original gain of the desired wave. Namely, it is impossible to execute an appropriate AGC control in an arrangement shown in FIG. 23.